Exhaust gas aftertreatment systems for internal combustion engines may include, for example, a particulate filter (PF) for removing particulates from the exhaust gas stream, such as soot from diesel exhaust. The most widely used particulate filters are wall-flow filters. A conventional wall-flow filter includes a ceramic honeycomb body having longitudinal, generally parallel cell channels formed by a plurality of intersecting porous walls. The cell channels are typically plugged with a ceramic plugging cement to form a checkered pattern of plugs at the ends of the honeycomb body. The cell channels of the filter typically have some ends unplugged at an inlet end of the honeycomb body, referred to herein as “inlet channels.” Likewise, typically, the cell channels also have the remaining ends plugged to form a checkered pattern of plugs at an outlet end of the honeycomb substrate with some ends unplugged, herein referred to as “outlet channels.” In use, exhaust gas containing entrained soot particles enters into the (unplugged) inlet channels, flows through the porous walls (i.e., the wall-flow) and into the outlet channels, and exits through the (unplugged) outlet channels, wherein the porous walls retain a portion of the particles that were previously entrained in the exhaust gas.
In conventional wall flow filter designs, every channel is plugged at alternate ends. In this conventional design, exhaust gas enters open channels on the inlet end. The inlet channels are plugged at the outlet end, and so the gas is forced to travel through the porous wall into an adjacent channel which is open at the outlet end but plugged at the inlet end to exit the filter. Filtration of the particulate matter is accomplished as the gas is forced to pass through the porous wall. Filtration efficiencies greater than 90% have been realized with conventional wall-flow filters.
Conventional wall-flow filters may be cleaned to prevent the filter from becoming blocked and to maintain a suitable pressure drop across the filter below a prescribed limit. Increase in pressure drop across the filter generally results in an increase in backpressure against the engine which, if not controlled, may lead to undesirable power loss. One known method for cleaning the filter is to remove the soot trapped in the filter by thermal regeneration (hereinafter “regeneration”). The regeneration may be either “passive” or “active” or a combination thereof. In “passive” regeneration, the inlet temperature of the exhaust gas entering the filter is sufficiently high to itself initiate combustion of the soot trapped in the wall-flow filter on a generally continuous basis, once steady state engine operating conditions are met. In “active” regeneration, the location of the filter is such that the temperature of the filter is relatively low and additional energy input may be required to raise the temperature of the exhaust (and the filter) to a level that causes combustion of the soot trapped in the filter. Typically, the additional energy input is provided by post injection of fuel into the exhaust in combination with an oxidation catalyst located upstream of the filter.
Exhaust aftertreatment systems based on “active” regeneration have become the industry standard because they desirably operate at lower exhaust temperatures and assure suitable soot removal under different engine duty cycles by actively initiating regeneration. On the other hand, “active” regeneration comes with a fuel economy penalty. Further, conventional filters may exhibit relatively high back pressure. Accordingly, systems and filters which operate with fewer regeneration events during operation are desired, as are filters exhibiting lower backpressures.